“It just little by little gets worse and worse,” Johnson told The Associated Press. “My body is to the point where I need time to rest and retire so I can be healthy for the rest of my life. It’s hard to wrap my mind around. Gymnastics has been my entire life, and now it’s no more.”

Johnson, who is now 20 years old, made the announcement just four days before the start of the U.S. gymnastics championships. Her competitive career, which took her from Iowa to Hollywood, with a few world titles, a trip to Beijing and an Olympic gold medal in between, is sadly over.

The petite gymnast with a bubbly personality won four medals — three silvers and a gold — with the U.S. team during the 2008 Beijing Games and returned home a celebrity athlete, signing numerous endorsement deals and winning the television competition show Dancing with Stars.

Johnson left the door open to a return for a London but blew out her knee in a 2010 ski accident, tearing her ACL, MCL meniscus and hamstring in one fell swoop. Despite the setback, she and her coach Liang Chow began plotting a comeback.

She made the team for last year’s Pan American Games, where she helped the Americans win the team gold. But when she tried to increase her training over the last few months to get ready for London, her surgically repaired knee just couldn’t cope with a rigorous Olympic training regimen,

Finally, Chow sat Johnson down and said they needed to be realistic. She couldn’t put in the training she needed, and she was looking at an entire knee reconstruction if she kept going.

“It’s been a really hard decision. How can you tell yourself, `No, I think it’s time to say it’s finally done?'” said Johnson, 20. “I’d like to be 30 and have kids and run around with them. It became more about my future life than this future one moment. I’m looking at the bigger picture of things.”

But it still hurts.

“It’s weird, for the first time in my career I came up short. But I feel like I succeeded as well,” said Johnson, who listed making the Pan Am team after the knee injury as one of her proudest accomplishments. “It almost came too easy the first time. It was a humbling experience this time around.”

Famous is the story of how her coach got Johnson her first invitation to a national team training camp. Unsolicited, Chow sent national team coordinator Martha Karolyi a highlight tape and promised that “this kid will help the U.S. team.” Based on the pure moxie of the move, Karolyi couldn’t help but take a look. Chow turned out to be right.

“Shawn will always hold a special place in gymnastics and my heart,” Karolyi said. “She always showed the joy of doing gymnastics.”

Along with her gold on balance beam in Beijing, Johnson won silvers in the all-around, team competition and floor exercise. Her all-around title at the 2007 world championships was at the time the fourth by a U.S. woman, and she also led the Americans to their third team title and took gold on the floor exercise. Shannon Miller is the only other American to win three golds at a single world championships.

Johnson’s performance at worlds was the exclamation point on a winning streak the likes of which is rarely seen in gymnastics. She won every event she entered in 2007, establishing herself as one of the poster kids for the 2008 Olympics. It also set up a compelling, yet friendly, rivalry with fellow gymnast Nastia Liukin.

After finishing second to Liukin at the Beijing Games, Johnson was among the first to embrace her teammate and congratulate her. If she was disappointed, she wasn’t going to spoil Liukin’s accomplishment by showing it.

Johnson said she made her decision to call it quits last Friday, then spent the next day telling those closest to her. But no matter how many times she said the words, they hadn’t quite sunk in. It will no doubt be difficult over the coming months, as she watches the rest of the U.S. women fight for a spot on the Olympic team and experience everything she did just four years ago.

She still plans to attend the Olympic trials where hopefully her training partner, Gabby Douglas, will make the U.S. team. And she also plans to go to London, where she promised to be “the greatest cheerleader in the stands.”

Her autobiography, “Winning Balance,” is being released Tuesday, and she plans to go to college in the fall of 2013, though she hasn’t settled on a school.

Though her comeback may not have ended how she imagined or hoped, Johnson said she has no regrets. She felt she had gotten “off track” in the two years after Beijing, and returning to gymnastics reminded her of what was really important.

“The comeback has made me 10 times more a stronger person than I ever was,” she said. “The past two years in gymnastics brought me back to the real me. I feel like no matter where I go next, I’ll be more level-headed about it. And do what’s right for me.”

So all you gymnastics fans out there will have to rearrange the brackets you’ve been working on. With Johnson out, who will make the U.S. team?

And as a reminder of how good Johnson was, take a look at the video from her winning performance as a 15-year-old phenom at the 2007 World Championships.